1. Field of the Invention
The present invention generally relates to a luminance/chrominance separator for separating a luminance signal from a chrominance signal in a composite video signal as one of television signals.
2. Description of the Related Art
FIG. 21 shows a first example of conventional luminance/chrominance separators. In the drawing, reference numerals 2102 and 2103 denote one line (1H) memories, numeral 2104 denotes a correlation detection circuit, numeral 2105 denotes a comb filter A, 2106 denotes a comb filter B, 2107 denotes a band pass filter (BPF), 2108 denotes a selection circuit, and 2109 denotes a high pass filter (HPF), respectively.
With such a conventional luminance/chrominance separator arranged as mentioned above, when the separator receives an NTSC signal from a terminal 2101, the 1H memory 2102 receives an input signal S1 and outputs an output signal S2 while the 1H memory 2103 receives the input signal S2 and outputs an output signal S3. Shown in FIG. 20A is a relationship between these pixels S1, S2 and S3 in position and the phase of a color subcarrier. As illustrated, the phase of the color subcarrier is inverted for every line.
The comb filter (A) 2105 subtracts the signal S1 from the signal S2 and outputs a signal C1 containing a chrominance signal and low-pass luminance signal, the comb filter (B) 2106 subtracts the signal S3 from the signal S2 and outputs a signal C2 containing the chrominance signal and the low-pass chrominance signal, and the BPF 2107 outputs a signal C3 having the color subcarrier as its center.
The correlation detection circuit 2104 detects a correlation between the processing pixel S2 and the adjacent pixels S1 and S3 and outputs a signal indicative of an optimum filter to the selection circuit 2108. The detection of the correlation and the selection of the optimum filter are carried out, as an example, in the following manner.
That is, the correlation detection circuit 2104 compares the absolute values of high-frequency components of the signals S1, S2 and S3. When the correlation detection circuit 2104 determines that the signal S2 is closer in magnitude to the signal S1, the signals S2 and S1 have a strong correlation therebetween and the comb filter (A) 2105 is selected as the optimum comb filter. When the signal S2 is close in magnitude to the signal S3, the signals S2 and S3 have a strong correlation therebetween and the comb filter (B) 2106 is selected as the optimum one. Further, if a difference between the signals S2 and S1 or S2 and S3 is greater than a threshold, then the signal S2 has no correlation with the signals S1 and S3 so the BPF 2107 is selected as the optimum one. In this way, the selection circuit 2108 selects optimum one of the filters depending on the output of the correlation detection circuit 2104 and then the low-frequency components are removed from the signal C1, C2 or C3 through the HPF 2109 to obtain a chrominance signal. The chrominance signal is subtracted at a subtraction circuit 2110 from the original signal S2 to obtain a luminance signal.
When such a signal as shown, for example, in FIG. 19A is applied to this conventional luminance/chrominance separator, since this signal is switched from red to black at a boundary line, one of the comb filters using strongly correlative pixels is selected for its separation without deterioration.
Variations in the signal of FIG. 19A at the boundary are shown in FIG. 20B, wherein (L-1) and L line signals indicate both red signals, there is a boundary between L and (L+1) lines, and an (L+1) line signal contains no chrominance signal because it indicates black. Under such a condition, when it is desired to separate the L line signal from the input signal, it is judged that the L and (L-1) lines have a strong correlation therebetween because the L line signal is equal in absolute value to the (L-1) line signal. Since the L line signal is not equal in absolute value to the (L+1) line signal, however, the L and (L-1) line signals are used to carry out the luminance/chrominance signal separation.
Meanwhile, when such a signal as shown in FIG. 19B is applied to the luminance/chrominance separator of FIG. 21 having such an arrangement as mentioned above, the applied signal has a curved boundary which slant part varies smoothly. For this reason, signals at the boundary are shown by FIGS. 13A, 13B and 13H. In the illustrated drawing, the signals each represent a half cycle and have a chrominance signal gradually increased in magnitude from the (L-1) line. Accordingly, the prior art luminance/chrominance separator of FIG. 21 judges that the pixel S2 has no correlation with its upper and lower pixels and thus the BPF 2107 is frequently used. However, when the BPF 2107 is used with respect to such a part of the input signal having a weak correlation as an edge part or an upper or lower part, this results in that a high-frequency luminance signal is lost and the resultant image becomes blurry.
When the BPF 2107 is used for a less amount of part of the input signal for the purpose of preventing such a blurry image, this disadvantageously involves the increase of dot disturbance.
In order to solve the above disadvantage, there has been suggested a second conventional luminance/chrominance separator as shown in FIG. 22 (refer to JP-A-62-145992).
The luminance/chrominance separator of FIG. 22 includes an input terminal 2000, one-line (1H) memories 2001 and 2002, minus 1/4 multipliers 2003 and 2005, a 1/2 multiplier 2004, adders 2006 and 2009, a band pass filter (BPF) 2007 for extracting a signal having frequencies in the vicinity of the frequency of a subcarrier, field memory (1F) 2008, a trap circuit 2010 for suppressing the signal having frequencies in the vicinity of the frequency of the subcarrier, a high pass filter (HPF) 2011, an absolute value circuit (ABS) 2012, an amplitude comparison circuit 2013, an amplitude limiting circuit 2014 and a subtracter 2015.
With the second conventional luminance/chrominance separator having such an arrangement as mentioned above, when a discrete NTSC signal S1 is applied to the separator from the terminal 2000, the input signal is delayed at the 1H memories 2001 and 2002 to obtain a signal corresponding to 3 lines.
Assume that the processing signal is an output S0 of the 1H memory 2001. Then a signal before one line corresponds to an output S2 of the 1H memory 2002 and a signal after one line corresponds to the input signal S1. These signals S0, S1 and S2 are applied to the 1/2 multiplier 2004, -1/4 multiplier 2003 and -1/4 multiplier 2005 respectively and outputs of these multipliers are then applied to the adder 2006. The multipliers 2003, 2004 and 2005 and the adder 2006 make up a comb filter. Since the chrominance signal in the NTSC signal is inverted for every line, the output of the adder 2006 comprises almost a chrominance signal because luminance signal components which have a correlation between three lines are canceled each other.
The output of the adder 2006 is further applied to the BPF 2007 which reduces luminance signal components which have little correlation between three lines.
The processing signal S0 is also applied to the field memory (1F) 2008 and then to the adder 2009 where a signal delayed by one field is added to the signal S0 not subjected to the delaying operation. The field memory 2008 comprises, more precisely, a 262-line delay circuit. Thus, the phase of a color subcarrier in the output of the 1F memory 2008 is inverted with respect to the phase of its input signal. For this reason, the output of the adder 2009 contains no color subcarrier because it is canceled each other at the adder 2009.
Further, the output of the adder 2009 is subjected at the trap circuit 2010 to a suppression of its color subcarrier components, subjected at the HPF 2011 to an extraction of its high-frequency components, subjected at the absolute value circuit 2012 to a calculation of absolute value of the output of the HPF 2011, subjected at the amplitude comparison circuit 2013 to an amplitude comparison between a reference value and the absolute value, and then subjected at the amplitude limiting circuit 2014 to a limitation of amplitude of the aforementioned chrominance signal output of the comb filter. The circuit of FIG. 22 judges whether the chrominance signal output of the comb filter corresponds to the original chrominance signal or a cross color noise signal (leakage of the luminance signal into the chrominance signal) and on the basis of such a decision result, adjusts the chrominance signal output.
Explanation will next be made as to the operation of the second prior art luminance/chrominance separator by referring to FIGS. 23-25.
FIG. 23 (A) shows a luminance signal at the current scanning line, FIG. 23(B) shows an output of the HPF 2011 and FIG. 23 (C) shows the output of the absolute value circuit 2012. Since the signal of FIG. 23 (A) has no high-frequency component, luminance/chrominance separation can be carried out from the signal and no cross color noise occurs without use of any comb filter. Accordingly, the output of the absolute value circuit 2012 is as shown in FIG. 23 (C), i.e., is below a reference level S and thus a chrominance (C) signal (in this case, no chrominance signal exists) is output from the amplitude adjustment circuit 2014 as it is.
When the luminance signal is as shown in FIG. 24 (A), the luminance signal has a high-frequency component at its edge part and thus the high-frequency component of the luminance signal leaks into the chrominance signal. In this case, the output of the absolute value circuit 2012 exceeds a threshold level S at its edge part as shown in FIG. 24 (C). Thus, the amplitude limiting circuit 2014 suppresses the leaked cross color signal at the edge part.
When the high-frequency component of the luminance signal has a frequency close to that of the chrominance signal as shown in FIG. 25(A), the output of the absolute value circuit 2012 exceeds the threshold level S as shown in FIG. 25(C), even in which case the unnecessary cross color interference or noise signal (corresponding to part of the luminance signal leaked into the chrominance signal) is suppressed at the amplitude limiting circuit 2014.
However, in the case where a moving picture signal is applied to the second prior art luminance/chrominance separator, no leakage of the luminance signal into the chrominance signal but the adder 2009 outputs a signal, which results in that, in spite of the fact of no leakage of the luminance signal into chrominance signal, the chrominance signal is suppressed.
In addition, the prior art luminance/chrominance separator has had such a problem that the separator can suppress the leakage of the luminance signal into the chrominance signal but cannot suppress the reverse leakage of the chrominance signal into the luminance signal.